What Is Energy Efficiency? 

When one form of energy is converted into another, some of the energy initially available is dissipated. 

Example for a washing machine: 

It consumes a certain amount of electrical energy = energy consumed. 

Some of this energy is converted into

mechanical energy

Sum of the potential energy and kinetic energy of an object or system. Potential energy is th...

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= useful energy. 

The rest is lost as heat = energy lost.

The efficiency of this washing machine = the relationship between the useful energy and the energy consumed by the machine. 

Calculating it as a % is simple: Efficiency (η) = (E_out)/(E_in) x 100 

η: Efficiency 

E_out: Useful energy 

E_in: Energy consumed 

Example: The efficiency of an incandescent light bulb is around 5%. This means only 5% of the energy consumed by the bulb is converted into useful energy, i.e., producing light. 95% of the energy consumed is lost as heat. 

The efficiency of an LED lamp is more than 30%. It is therefore more efficient, because less energy is lost. Efficiency is key data for calculating energy performance. 

Energy performance = average energy efficiency of a system (e.g., a building). 

To assess this, we must calculate the average of several representative efficiency readings (heating, insulation, air conditioning, lighting).
 

Summary:

• The efficiency of a device or system = the relationship between the useful energy given out and the energy consumed. 
• Energy consumed = total amount of energy consumed by the device. 
• Useful energy = proportion of energy consumed for the desired purpose.
• Efficiency is key data for calculating energy performance (e.g., of a building).